A silicon carbide (SiC) single crystal has superior properties such as thermal and chemical stability, excellent mechanical strength, good resistance to radiation, and dielectric breakdown voltage and thermal conductivity higher than Si. It is also characterized in that it is easy to electronically control p and n conductivity types by doping an impurity, as well as it has a wide band gap (about 3.0 eV for single crystal 6H—SiC, about 3.3 eV for single crystal 4H—SiC). Therefore, it can achieve high temperature, high frequency, resistance to voltage and resistance to environment, which cannot be achieved by any existing semiconductor material, such as silicon (Si) and gallium arsenide (GaAs). It is increasingly expected as a next-generation semiconductor material.
Heretofore, a vapor phase method, Acheson method, and a solution method are known as typical methods for growing a silicon carbide single crystal.
Typical examples of vapor phase methods include a sublimation method and a chemical vapor deposition (CVD) method. In a sublimation method, various defects tend to be formed in a crystal and a crystal tends to be poly-crystallized. In a CVD method, because raw materials are limited to gases, a formed crystal is a thin film and therefore it is difficult to produce a bulk single crystal.
Furthermore, in Acheson method, since silica stone and coke are used as raw materials and heated in an electric furnace, it is impossible to obtain a high-purity product because of impurities contained in the raw materials.
And, the solution method is a method that comprises melting silicon-containing alloy in a graphite crucible, dissolving carbon from the graphite crucible into the melt, growing silicon carbide crystal layer on a seed crystal substrate placed at the cold area by solution deposition. And, it is known that the solution method has low growth rate but it is advantageous as a method to obtain a bulk crystal.
For this reason, recently, various studies have been done to enhance a growth rate for growing a silicon carbide single crystal by a solution method that has not above-mentioned problems in a vapor phase method and Acheson method.
Japanese Unexamined Patent Publication No. 2000-264790 describes a method for producing a silicon carbide single crystal, wherein the method comprises melting a raw material containing at least one element of transition metals, Si, and carbon to form a melt, bringing the melt into contact with a silicon carbide seed crystal as well as cooling the melt to the temperature lower than the liquidus-line temperature of the melt, depositing and growing the silicon carbide single crystal. And, although it discloses Fe, Co, Ni (group VIII), Ti, Zr, Hf (group IVb), V, Nb, Ta (group Vb), Cr, Mo and W (group VIb) as transition metals, it discloses only a composition comprising Mo, Cr, and Co. However, with respect to the quality of a depositing single crystal, a measuring method or identifying means is not disclosed, and a macro defect of the crystal growth surface is not recognized.
Japanese Unexamined Patent Publication No. 2004-2173 describes a method for producing a silicon carbide single crystal, comprising the steps of immersing a seed crystal substrate of silicon carbide into an alloy melt that contains Si, C and M, where M is Mn or Ti, has M/(Si+M) atomic ratio (X) of 0.1 to 0.7 when M is Mn and 0.1 to 0.25 when M is Ti, and does not contain unmelted C, and growing a silicon carbide single crystal on the seed crystal substrate by putting silicon carbide into super-saturation state by super-cooling the alloy melt in the periphery of the seed crystal substrate. In addition, it described that silicon carbide tends to be poly-crystallized by carbon that was charged as a raw material, concerning the method for producing a silicon carbide single crystal described in Japanese Unexamined Patent Publication No. 2000-264790.
Japanese Unexamined Patent Publication No. 2006-143555 describes a method for producing a silicon carbide single crystal, wherein the method comprises immersing a seed crystal substrate of silicon carbide into a alloy melt that contains Si, C and M, where M is Fe or Co, and has the value of [M]/([M]+[Si]), wherein [M] expresses molar concentration of M; [Si] expresses molar concentration of Si, not less than 0.2 and not more than 0.7 when M is Fe, or not less than 0.05 and not more than 0.25 when M is Co, growing a silicon carbide single crystal on the seed crystal substrate by putting the alloy melt in the periphery of the seed crystal substrate into super-saturation state of silicon carbide. However, a macro defect of the crystal growth surface is not recognized.
Japanese Unexamined Patent Publication No. 2007-76986 describes a method for producing a silicon carbide single crystal, wherein the method comprises bringing a single crystal substrate for growing silicon carbide contact with a melt that contains Si, Ti, M and C, where M is Co and/or Mn and has the following atomic ratio of Si, Ti and M: 0.17≦[Ti]/[Si]≦0.33 and 0.90≦([Ti]+[M])/[Si]≦1.80, where [Ti] expresses molar concentration of Ti, [M] expresses molar concentration of M, or that contains Si, Ti, M and C, where M is Al, and has the following atomic ratio of Si, Ti and M, wherein, 0.17≦[Ti]/[Si]≦0.33 and 0.33≦([Ti]+[M])/[Si]≦0.6, where [Ti] expresses molar concentration of Ti, [Si] expresses molar concentration of Si, [M] expresses molar concentration of M, growing a silicon carbide single crystal on the single crystal substrate by putting the silicon carbide dissolved in the melt into super-saturation state by super-cooling the melt in the periphery of the single crystal substrate. However, the macro defect of a grown crystal surface is not recognized.